To alleviate an energy crisis caused by insufficient energy supply due to a continually increased energy demand and to mitigate influences on the ecology of the earth associated with conventional power generation from fossil fuels or nuclear power, there is a growing consensus on the use of sunlight as an alternative energy source since the solar energy is unlimited, clean, and ubiquitous.
Photovoltaic (PV) solar cells make it possible to convert the sunlight into electricity. However, a wide use of solar power generation depends on technology advances that lead to higher efficiency, satisfied reliability, and lower cost, thus making it more competitive with the conventional power generation methods. One of the most important and costly aspects of solar cell manufacture is the way of contact metallization (also called “electrode”), which provides electrically conducting paths on the surface of cell to collect and transfer photo converted charges to an external circuit, thus generating useful electrical energy. The reliability and efficiency of solar cells are largely affected by the nature and quality of the metal contacts.
Crystal silicon solar cells constitute an important technology in production. Crystalline solar cells are fabricated using either mono-crystalline silicon or multi-crystalline silicon as substrates. These substrates are commonly modified with a dopant of either positive or negative conductivity type, and are on the order of 50-500 microns in thickness. The surface of the substrate, such as a wafer, intended to face incident light, is designated as the front surface and the surface opposite the front surface is referred to as the back surface. By convention, positively doped silicon is commonly designated as “p”, where holes are the majority electrical carriers. Negatively doped silicon is designated as “n” where electrons are the majority electrical carrier. The key to the operation of a photovoltaic cell is the creation of a p-n junction, usually formed by further doping a thin layer at the front surface of the silicon substrate. Such a layer is commonly referred to as the emitter layer, while the bulk silicon is referred to as the absorber layer. The emitter may be either p-doped or n-doped depending on the configuration of the device.
Electrodes are formed on the front and back of crystalline silicon PV devices. Electrodes on the front are typically formed using metal pastes, composed primarily of silver and silver alloys. These electrodes are deposited in arrays on the front surface of the device using techniques such as screen printing. The substrate is typically the silicon solar cell covered by a dielectric film, typically silicon nitride, that serves as an antireflective coating. Electrodes on the front or back side of the device have the requirement of high conductivity and low contact resistance.
Currently, there is a global trend in the PV industry toward developing thin film solar cells. The reason is that wafer based solar cells are difficult to mass-produce and cannot be provided inexpensively, as they require much time and energy for crystal growth and complex subsequent manufacturing steps. Amorphous silicon type solar cell is expected as one of the most promising thin film solar cells, because it has excellent characteristics such as the operability with thin films due to its great light absorption coefficient and the easy formation in large area on a low-cost or flexible substrate through relatively simple manufacturing steps.
The photoelectric conversion layer in thin film solar cells contains at least one or more p-i-n junctions and the stack of the active layer is normally microns thick, while in conventional wafer based solar cells, the active layer is typically hundreds of microns thick. Therefore, the sheet resistance of the active layer in thin film solar cells is much higher than that in wafer based cells. The high sheet resistance of the active layer retards lateral charge transfer during charge collection by front electrode. While, an increase in the density of grid lines on the sun-facing front face of the solar cell will deteriorate the shading effect on the photovoltaic junction and reduce cell output. To compensate this drawback of thin film active layer, the front electrode of thin film solar cells is generally a transparent conductive oxide (TCO), such as indium tin oxide (ITO) or zinc oxide, which enables incident light to reach the light absorbing material and serves as an ohmic contact to collect electrical charges converted there from the light radiation. The TCO also acts as an anti-reflective coating (ARC) layer. Since the resistance of TCO is intrinsically high compared with those of thick film made charge collectors, TCO must be made hundreds of nanometers to microns thick in order to create an electrode layer with a sufficient conductivity. Even though, metal grid lines are still required to be added on the TCO surface to further assist in charge collecting. An intimate contact between metal in the grid lines and TCO surface is highly desired to ensure the efficiency of the charge collecting.
Several methods are known for electrode formation. Vapor deposition or sputtering to form a conductive metal can form electrodes, but suffers from the drawback that these methods are not cost effective on large-area cell devices. One method for forming electrode on thin film and silicon wafer based PV solar cells is using screen printed thick film technology.
The quality of amorphous silicon can be easily damaged when processing temperatures subsequent to the formation of the active layers is at or above 250° C. For this reason, polymer thick film (PTF) method is desirable for forming metal grid lines. PTF pastes may contain metal particles, polymer resins, and other additives. The pastes are deposited on the TCO surface by screen printing into a grid line pattern and then curing at a relatively low temperature, such as below 250° C. After curing, metal particles are physically connected each other and fixed by a polymer matrix, thus forming a conductive film. The polymer resins or binders also provide an adhesion between PTF and TCO. However, charge collecting electrodes manufactured by these known methods suffer from the drawback of a decrease in conversion efficiency and are not satisfactory in the reliability in the prolonged use as compared with those electrodes made of metal thick film method. The drawbacks for electrodes formed from these methods include the following:                1) The resistivity of such types of collecting electrodes (approximately 30 to 50 μΩ-cm) is significantly higher than that made by metal thick film on c-Si based cells (5 to 8 μΩ-cm), leading to an increase in Joule loss and this increases the loss of conversion efficiency.        2) Polymer binders do not act as barriers to oxygen. The resistivity of the electrode increases with time due to a continual oxidation on the metal particle surface, which largely reduces the electrode durability against temperature and humidity.        3) The curing time of PTF is normally long (e.g. 30 minutes to one hour or more), which largely increases the process cycle time.        4) The solderability of the formed PTF electrode is normally poor due to insufficient and embedded metal particles.        5) Some material in PTF, such as a polymer binder, can optically degrade upon irradiation with light to cause breaking of the electrode and peeling off of the electrode from TCO surface, thus making the electrode to lose its function.        6) It is the surface oxidation of metal particles in the existing PTF technology that inhibits the use of most metals as conductive fillers. Only noble metals, such as silver, are widely used as a conductive filler material in PTF, which not only increases the material cost but also brings a silver migration problem.        
Generally speaking, copper is more desirable than silver as a printed conductor due to its low material cost and favorable electrical properties, such as higher electro migration resistance and lower self diffusivity. However, copper differs from silver because it normally has a relatively thick and non conductive native oxide layer on the surface and it has a high oxidation potential, which are significant technical challenges associated with switching silver by copper for most of printed electronics applications. Therefore, the use of copper thick film for front-side contacts on thin film PV solar cells is not generally known.
Following efforts have been made by previous researchers to solve the oxidation induced degradation of electrical performance. Dielectric encapsulates have been applied to adhesives to form an oxidation barrier, but this only retards rather than prevents oxidation. Oxide-reducing agents have been added to adhesives and this approach is only effective over a short period of time. Acid anhydride epoxy hardener has also been used. The epoxy can initially reduce surface oxides, but continual aging causes the oxide to reform. Using a metal alloy powder with a low melting point as a binder to bond Ag particles into a network structure in the conductive adhesive has been pursued. However during soldering, the formed network structure can be destroyed and metal alloys have an intrinsically low electrical conductivity.
As described above, typical PTF conducting materials are made of individual particles which are physically connected each other. In contrast, PARMOD® materials disclosed by Parelec Inc. provide “chemical welding” of pure metal particles including copper. These materials are disclosed in, for example, in U.S. Pat. No. 5,882,722, U.S. Pat. No. 6,036,889, U.S. Pat. No. 6,143,356, U.S. Pat. No. 6,153,348, U.S. Pat. No. 6,274,412 B1, U.S. Pat. No. 6,379,745 B1, which are hereby incorporated by reference in their entirety. Such formed electrical conductors are in a continuous network structure, thus eliminating the oxidation induced degradation of electrical performance. In addition, PARMOD® materials can be consolidated at relatively low temperatures (e.g., 220° C.) in a forming gas environment (e.g., 5% H2 in N2) with an electrical resistivity better than that of Ag PTF (e.g., 21 μΩ-cm for PARMOD® copper versus 30 to 50 μΩ-cm for Ag PTF).
PARMOD® copper technology includes using a carboxylic acid, such as neodecanoic acid (NDA), to react with copper oxides on copper particle surface to form a copper neodecanoate (Cu NDA). The Cu NDA is then thermally decomposed into copper atoms during heating, which thus act as a “glue” to bond copper particles together into a continual network structure. Compared with conventional metal sintering, this copper sintering mechanism provides for lower process temperatures. However, the resistivity at the lowest achievable temperature for forming conductive copper(21 μΩ-cm, 220° C.) still fails to meet the expectation for making electrodes on thin film PV solar cells. The Cu NDA in situ formed during the consolidation process requires relatively high temperatures to decompose back to copper metal, thus rending the consolidation temperatures relatively high or conductivity of the formed copper relatively low for a given low consolidation temperature (e.g. 220° C.).
In addition, PARMOD® copper conductive material does not adhere well on commonly used substrates, such as polymers, papers, and TCO surfaces. Achieving sufficient adhesion without sacrificing the desired electrical resistivity in the conductive feature is challenging. Although attempts, such as U.S. 2004/0144958 A1, which is hereby incorporated by reference in its entirety, have been made to improve adhesion of the PARMOD conductive materials, a suitable solution to this problem has still not been developed, especially for copper.
U.S. Pat. No. 6,951,666, which is hereby incorporated by reference in its entirety, describes a screen printable metal precursor composition with a viscosity above 1000 centipoises for fabricating a conductive feature on a substrate. The composition contains a metal precursor compound, a liquid vehicle, metallic particles, and other additives, such as a polymer. During heating, the metal precursor compound will convert to metal thermally or thermal-chemically. However, the U.S. Pat. No. 6,951,666 patent doesn't teach forming non-precious metal conductors, such as copper conductors, having good adherence and low resistivity. Examples of copper compositions given in the U.S. Pat. No. 6,951,666 patent have poor electrical performance. For example as introduced in the U.S. Pat. No. 6,951,666 patent, the resistivity of the formed copper after a thermal consolidation at 350° C. is 24 times of bulk copper (example 30).
One method of metal consolidation useful for forming metal devices, including electrically conductive devices is sintering. Sintering is widely used to manufacture ceramics and high temperature metals. During conventional solid-state sintering, the porosity of a powder compact is continually reduced by mass transfer from the surface of particles to the neck area of the compact at a temperature below the melting point of the solid phase. The driving force of densification is a reduction of surface energy by reducing the surface area of the solid phase. The mass transfer is derived by the differences in surface curvature and consequently the differences in vapor pressure between the surface of the particle and the neck area. This conventional solid state sintering mechanism typically requires a temperature ranging from 650° C. to 900° C. and times ranging from minutes to hours at pressures tons per square inch. These high temperatures and pressures are not suitable for use with a substrate such as a thin film PV cell.
Another commonly used mechanism for metal consolidation is sintering in the presence of a liquid phase, where the solid powders have a certain limited solubility in the liquid at the sintering temperature. The driving force for liquid phase sintering is derived from the capillary pressure of the liquid phase located between the fine solid particles. The capillary pressure provides a force to continually rearrange particles to give more effective packing. The densification of solid phase is accomplished by a solution of smaller particles and growth of larger particles by mass transfer through the liquid phase. The densification process is slowed and stopped once a solid skeleton is formed. It is believed that in the conventional liquid phase sintering, dissolution rather than precipitation is normally the rate-limiting step for the formation of the skeleton structure especially at low temperature range. Precipitation, on the other hand, directly contributes to the formation of the skeleton structure, which increases with decreasing temperature and increasing metal concentration in the liquid phase.
Accordingly, there is an industrial need on improving contact metallization system of the solar cells, such as pursuing a lower material and manufacturing costs, better electrical performance, and more satisfied long term stability, and there is a need in the art to provide a novel thick film compositions comprising metals, particularly non-precious metals such as copper, and methods comprising same. More specifically, process temperatures of the copper thick film should desirably be maintained below 250° C., or more preferably, below 200° C. to prevent thermal damage to the amorphous silicon, a strong adhesion to TCO surface, a good solderability, and no copper contamination induced shunting defect.